Regulating coal slurry settling rates

ABSTRACT

The sizes and specific gravities of coal solids in a slurry are limited on the basis of particle settling rates to prevent undesirable settling of the solids in a coal slurry handling system, especially in a liquefaction reactor for liquefying slurried coal solids. After makeup with crushed coal particles in a suitable liquid carrier, a coal slurry is charged in a continuous process to a settling rate separation zone, preferably embodied in a hydrocyclone, where the slurried coal solids are segregated according to whether they exceed a predetermined settling rate limit, one which preferably is within the range from about 0.5 to about 300 ft./min., coal solids settling faster than the limit rate being separated for constitution in an unfinished heavy slurry stream from the slurry particles which do not settle, at maximum settling velocity, in excess of the settling rate limit, such particles issuing in a finished slurry stream as product from the settling rate separation zone. The unfinished heavy slurry stream is passed to a comminution zone, preferably a ball mill, where the coal particles in the stream are milled to reduce their sizes, at least a portion of the particles being reduced to sizes, which for the specific gravities of such particles, will permit the particles to settle in the separation zone at a rate not greater than the predetermined limit. The comminuted heavy slurry stream is then mixed with new slurry feed and recycled to the settling rate separation zone where the comminuted coal particles and the new feed coal solids which do not settle at a maximum velocity in excess of the predetermined limit are constituted as finished slurry product, the particles exceeding the limiting rate being cycled to the comminution zone for further treatment.

a United States Patent [72] Inventor Edgar (I. Winegartner ABSTRACT: Thesizes and specific gravities of coal solids in a Baytown, Tex. slurryare limited on the basis of particle settling rates to [21] Appl. No.34,233 prevent undesirable settling of the solids in a coal slurry han-[22] Filed May 4, 1970 dling system, especially in a liquefactionreactor for liquefying' [45] Patented Nov. 2,1971 slurried coal solids.After makeup with crushed coal particles [73] Assignee Esso Research andEngineering Company in a suitable liquid carrier, a coal slurry ischarged in a continuous process to a settling rate separation zone,preferably embodied in a hydrocyclone, where the slurried coal solidsare segregated according to whether they exceed a predetermined settlingrate limit, one which preferably is within the range from about 0.5 toabout 300 ft./min., coal solids settling faster than the limit ratebeing separated for constitution in an un- [54] REGULATING COAL SLURRYSETTLING RATES- 6 Claims, 1 Drawing 8- finished heavy slurry stream fromthe slurry particles which do [52] US. Cl 241/16, Settle at maximumSewing velocity in excess of 241/29 tling rate limit, such particlesissuing in a finished slurry [51] Int. Cl. B02c 23/00 stream as Productfmm the settling rate separation zone The 501 Field of Search 241/15,16, unfinished heavy is Passed a cmminufin 19 20, 21,29 zone, preferablya ball mill, where the coal particles in the stream are milled to reducetheir sizes, at least a portion of the [56] Ref ren e Cited particlesbeing reduced to sizes, which for the specific gravi- UNITED STATESPATENTS ties of such particles, will permit the pfiirticllfs to sgttlein th;

se aration zone at a rate not greater t an t e re etermine gigs g: lil'fl il. The comminuted heavy slurry stream is tlFen mixed with 3 524:6828/1970 35mm 241/15 x new feed and "cycled the 8mm; me sepmm PrimaryExaminer-Granville Y. Custer, Jr. Attorneys-Timothy L. Burgess, ThomasB. McCulloch,

Melvin F. Fincke, John S. Schneider, Sylvester W. Brock,

zone where the comminuted coal particles and the new feed coal solidswhich do not settle at a maximum velocity in excess of the predeterminedlimit are constituted as finished slurry product, the particlesexceeding the limiting rate being cycled Jr. and Kurt S. Myers to thecomminution zone for further treatment.

COARSE COAL CRUSHER CONDENSER CYCLONE WATER l5 l2 E LIQUEFACTION ZONESURGE sit as}; 3:

SLURRY HEATER coulieron DMER 3-3 24 3O FINISHED 23 SLURRY 20 2:3

SEPARATION zone 2s 1 l BALL MILL PATENTEDuu'v 2 ISYI COAR SE COALCRUSHER '/'CYCLONE SURGE B IN CONVEYOR CONDENSER SLURRY DRYER BALL MILLWATER LIQUEFA ZONE ' HEATER FINISHED CTION SLURRY i SEPARATION ZON INVENTOR.

Edgar C. Wlnegortner,

BY$WXWM ATTORNEY.

1 REGULATING COAL SLURRY SETTLING RATES BACKGROUND OF THE INVENTION Thisinvention is directed to the processing of coal slurries to controlexcessive particle settling and plugging in slurryhandling systems. Moreparticularly, it involves the processing of coal slurries to restrictthe particle makeup of the slurries to solids which do not settle atrates exceeding a predetermined limit.

When handling coal slurries in systems such as slurry transportpipelines and slurry-treating reactors, it is necessary to constitutethe slurries so that the slurried coal solids do not settle out ofsuspension and plug up the slurry-handling system. Particle settling ina slurry is a function of the size and specific gravity of the particle.On the assumption that the specific gravity of the particle material isconstant, i.e., that the differences in specific gravity of solids inthe slurry are negligible, efforts heretofore to control settling inslurry handling systems have been based on sizing operations. Thus, thedesign of slurry-handling systems has specified that slurries handled bythe system be constituted of particles smaller than a given size. (Seee.g. Manno, P. .I., Slurry Pipelines, Stanford Research InstituteReport, 1967; Wicks, M., Optimization of Solids Concentration andParticle Size Distribution," 22 nd Technical Meeting of South TexasSection of AIChE, Oct. 13, 1967; and Wasp, E. J., et al., A Quick Methodof Making Initial Cost Estimates of Slurry Pipelines," AIME Paper 69865,AIME, Feb. 16-28, 1969, Washington D. C.) The assumption thatdifferences in specific gravities of materials in a slurry arenegligible can have serious consequences. A well-documented example isthe plugging of the 1.5 million tons per year Cadiz, Ohio to East Lake,Ohio coal slurry pipeline in 1957 because 0.01 weight percent of thecoal solids in the slurry were of higher specific gravity than the othersolids in the slurry, which was made up in accordance with designspecifications of solids finer than a specified size. In reactors suchas coal liquefaction reactors which are used to liquefy slurried coalparticles in processes for producing liquid hydrocarbon products fromsolid coal, neglect of the differences between specific gravities ofcoal particles in a slurry can produce excessive settling ofnonliquefiable solids and resultant plugging of the reactor, requiringfrequent and costly shutdowns for cleanout.

The specific gravity of coal particles in a slurry can be made uniformby sorting of the solids used to make up the slurry, but sortingoperations offer no size control of the solids, and consequently, do notsolve the problem of excessive settling in coal slurry handling systems.What is needed is a process which makes up coal slurries with regardboth to the size and the specific gravity of the constituent particlesso that undesired settling of the particles in a coal slurry handlingsystem is prevented. That is what this invention provides.

SUMMARY OF THE INVENTION In accordance with this invention, preventionof undesired settling of slurried coal particles in a coal slurryhandling system is achieved in a continuous process which entailspassing a slurry of the coal particles into a settling-rate separationzone where slurried coal particles are segregated according to whetheror not they settle at velocities which exceed a predeterminedsettling-rate limit, slurry solids exceeding the limit being separatedin an unfinished heavy slurry stream from slurry solids which at maximumsettling velocity do not exceed the limit, such solids being constitutedin a finished slurry stream issued as product from the separation zone.The unfinished heavy slurry stream is transferred to a comminution zonewhere slurried solids in the stream are milled or otherwise comminutedto reduce the sizes of at least a portion of the solids to sizes which,for the specific gravities of such solids, will provide the solids witha maximum settling rate less than the limiting settling rate of theseparation zone. The comminuted heavy slurry stream is then mixed withnew slurry feed for the separation zone, and the slurry mixture ispassed to the separation zone where new feed-slurried solids andcomminuted-slurried solids which do not settle at maximum settlingvelocities in excess of the predetermined maximum settling rate aresegregated from the remainder of the slurry solids in the zone and aredischarged from the zone as a finished slurry product, the particlesexceeding the limit being cycled in the unfinished heavy slurry streamto the comminution zone for further processing.

Aspects of the Invention The slurry which is processed according to thisinvention is made up of solid, particulate coal suspended in a suitableliquid carrier. The liquid carrier utilized will normally depend on whatis done with the slurry. If handling of the slurry involves only itstransport in a pipeline, the carrier liquid suitably may be water oroil. If handling of the slurry involves transport in a reactive system,such as when the coal slurry is treated in a liquefaction process toconvert the dissolvable portions of the coal into liquid form, theliquid carrier suitably may be a coal-derived oil, but preferablyis ahydrogen-donor solvent which boils withinthe range from about 300 F. toabout 900 F. at atmospheric pressure. Preferably, the liquid carrier hasa specific gravity within the range from about 0.90 to about 1.10.

The solid, particulate coal suspended in the liquid carrier may be ofany type or rank, depending on the purpose for which it is handled, butfor liquefactionprocesses, it preferably is bituminous coal,subbituminous coal, lignite, brown coal, or a mixture thereof. The coalparticles are suitably ground to a particle size distribution from about8 mesh (Tyler and finer, but there may be particles as large asone-quarter inch on the major dimension present in the slurry. In aliquefaction process, adequate conversion is obtained even with thelarger sized particles.

The specific gravity of a coal particle of given size will vary for anyrank of coal with the different maceral group constituents of the coal(vitrinites; exinites, including sporinites, cutinites, and resinites;and inertites, including micrinites, semifusinites, and fusinites) andtheir relative proportions in the coal, together with the kind andrelative proportions of mineral constituents in the coal. Common mineralconstituents include clay, felspar, kaolinite, calcite and pyrite. In aliquefaction process, it is particularly important to control thesettling of coal particles primarily consisting of nonliquefiableconstituents such as clay, pyrite, fusinite etc., for these are theconstituents which otherwise will tend to settle excessively in theliquefaction reactor and cause plugging. The specific gravity of a coalparticle is also affected by the extent to which it contains absorbedwater, and this depends on the constituency of the particular particle.In a liquefaction process. it is preferable to use coal which has driedto remove excess moisture, although it is feasible to employ coal whichis not moisture free if the liquefaction facilities have been sized toallow the withdrawal of evolved steam. In general, the specific gravityof coal particles will vary over the range from about 1.25 to about 1.4with mineral particles as heavy as 5.2 specific gravity also beingpresent.

The bulk viscosity and the bulk specific gravity of a slurry will varywith the kinds, sizes, specific gravity and concentration of coal in aslurry as well as the nature of the liquid carrier, i.e. its specificgravity, whether it be solvent or nonsolvent, hydrogenated or not etc.However, for handling in a liquefaction process, the slurry charged tothe separation zone, including recycled coal particles from thecomminution zone, will suitably have a bulk viscosity within the rangefrom about 1 to about 1,000 centipoise at 60 F., and also, a bulkspecific gravity within the range from about 0.95 to about 1.4 at 60 F.The solvent-to-coal ratio of the slurry charged to the separation zonewill suitably be within the range from about 0.8:! to about 2: l.

Settling-Rate Separation Zone.

The coal particles of the slurry charged to the separation zone areseparated according to whether their settling rate exceeds apredetermined limit in the settling-rate separation zone. In accordancewith Stokes law, the settling rate of individual particles in thesettling zone varies directly with the sizes of the particles, thedifference between the specific gravity of the particles and the bulkspecific gravity of the slurry and inversely with the bulk viscosity ofthe slurry. The limiting settling rate which is established to segregatethe particles of a particular coal slurry will be determined by theparticular handling system in which settling is to be controlled, theextent to which settling is to be avoided in that system, the velocityand throughput of a slurry in a time period in the handling system, thebulk viscosity and bulk specific gravity of the coal slurry charged tothe separation zone, and other factors. For a coal liquefaction processutilizing an unstirred upflow reactor, in which it is specified that allsettling is to be prevented in the reactor, and in which the coal slurrycharged to the separation zone is a 33 weight percent slurry of -20 meshbituminous coal in a hydrogenated creosote oil, and wherein the slurryviscosity is within the range from about 5 to about centipoise and theslurry specific gravity is within the range from about l.l5 to about 1.2at temperatures within the range from about 650 F. to about 950 F., thelimiting settling rate in the separation zone will suitably be withinthe range from about 5 to about 20 ft./minute. For a settling rate ofabout per minute in the same reactor under like conditions, one woulduse a slurry of l00 mesh coal to feed to settling-rate separation zone.For a settling rate of about 166 ft./min., the feed to the separationzone would be made up with -8 mesh coal. However, if the upflowliquefaction reactor is a well-mixed reactor, a 8 mesh coal may be used,for settling rates of about 300 ft./second are suitable. Accordingly,settling rates in the separation zone may suitably be within the rangefrom about 0.5 ft./minute to about 300 ft./minute, as desired for thesystem in which the slurry will be handled.

Preferably the settling rate separation zone is embodied in a liquidcyclone, or as it is more commonly called, a hydrocyclone. In this unit,commonly used for mineral dressing, washing, concentration, etc.settling velocities are increased by increasing the acceleration ofgravity such that the higher settling rate particles can be separated ina relatively short period of time. The use of a hydrocyclone to preparecoal slurries for handling in a transport system or a slurry-treatingreactor to avoid undesired settling is, to applicant's knowledge, whollynovel.

Comminution Zone.

As above described, particles settling in excess of the limiting rate inthe separation zone are constituted in an unfinished heavy slurry streamand transferred to a comminution zone. The comminution zone is suitablya ball, rod, or other suitable mill which will pulverizc, grind, mill orotherwise act upon the particles received from the separation zone toreduce the size of at least a portion of the particles to an extentwhich, for the specific gravity of such particles, will permit them tosettle in the settling zone at a maximum settling velocity not greaterthan the settling-rate limit when recycled from the comminution zone tothe settling zone.

In this closed circuit continuous size reduction and solids separationsystem, a circulating load is maintained between the separation zone andthe comminution zone, the load varying with the feed rate of new feed tothe separation zone and the rate at which a finished slurry isdischarged from the separation zone. Normally, the circulating load willbe in the range of IO percent to 300 percent ofthe fresh feed.

The invention will be better understood by considering the drawing inconnection with a description of a preferred manner ofcarrying out theinvention.

DESCRIPTION OF THE DRAWING The drawing is a schematic flow schemeillustrating the preparation of a slurry of coal particles, inaccordance with this invention, for use in a liquefaction reactor in acoal liquefaction process.

DESCRIPTION OF THE PREFERRED EMBODIMENT Coarse coal is introduced byline 10 into a sizing crusher ll, suitably a conventional hammer orimpact mill, in which the coarse coal is reduced in size to a desiredlevel, suitably percent through 8 mesh. The crusher 11 is locateddirectly above a receptacle or surge bin 12 which receives the crushedcoal from crusher 11. Where the crusher is a hammer or impact mill, thecrusher acts as a blower and transports considerable quantities of air,into the Surge bin 12. This air is conveniently removed from the surgebin by exhausting it through line 13 into a small conventional gascyclone 14, which drops out most of the particulate solids entrained inthe exhausted air and returns the solids to the surge bin by way ofconduit 15. The unladen exhaust is recycled to the inlet side of crusher11 by recycle line 16. In this manner, the quantity of fresh airentering the system is reduced and oxidation of coal during the crushingoperation is decreased.

Crushed coal from the surge bin 12 is fed to a slurry drier 17 by way ofa conveyor 18 for drying, preferably in accordance with the methoddescribed in my copending U.S. Pat. application Ser. No. 770,281, filedOct. 24, 1968, entitled Coal Drying. The slurry drier 17 contains a poolof an inert heat transfer liquid maintained at a temperature from about220 F. to about 600 F. The crushed coal is introduced into slurry drier17 and into the pool of heat transfer liquid at a coal inlet pointadjacent but below the surface of the heat transfer liquid. The heattransfer liquid is introduced into the slurry drier zone at a point nearthe coal inlet point. Moisture vaporized from the coal is vented fromslurry drier 17 by way of exhaust line 19, condensed, collected andultimately discharged. The crushed coal introduced into the slurry drierforms a slurry with the inert heat transfer liquid, preferably a hothydrogenated creosote oil or the like, and the slurry is withdrawn byline 20 from the bottom of the slurry drier, preferably as a 30 to 40weight percent solids slurry.

In accordance with this invention, the slurry from slurry drier 20 ischarged, as by slurry drier pump 21, into a maximum settling rateseparation zone 22, where the slurried coal particles are segregatedaccording to whether they exceed a settling-rate limit or not, slurriedsolids settling at rates exceeding the limit being separated in anunfinished heavy slurry stream from the slurry particles which atmaximum settling velocity do not exceed the limit, these particles beingconstituted in a finished slurry product. Preferably, the maximumsettling rate separation zone is embodied in a hydrocyclone. Thus, theslurry feed is introduced into a hydrocyclone 22 by way of a tangentialfeed inlet 23, the finished slurry product constituted of particleshaving a settling rate less than the predetermined limit is dischargedfrom the cyclone 22 by way of main fluid outlet 24, and the unfinishedheavy slurry stream consisting of particles exceeding the settling ratelimit is issued from cyclone 22 by peripheral fluid outlet 25.

The unfinished heavy slurry stream includes all particles larger than 8mesh plus some high specific gravity particles finer than 8 mesh in thecase where the crusher 11 is operated to reduce the coarse coal to 90percent through 8 mesh. The unfinished heavy slurry is transferred byline 26 to a comminution zone 27, preferably a ball mill, where theslurried solids in the heavy stream are milled to reduce the sizes of atleast a portion of the solids to sizes which, for the specific gravitiesof the solids, will provide the solids with a maximum settling rate lessthan the predetermined settling rate limit of the settling zone. Thedischarge from the comminution zone is withdrawn by line 28 and recycledback to the suction side of the slurry drier pump 21, mixing thecomminuted heavy slurry stream with new slurry feed from the slurrydrier. A slurry mixture of new feed and comminuted slurry solids is thuscharged back through the hydrocyclone 22, which issues the comminutedslurry solids and new feedslurried solids which do not settle at amaximum velocity in excess of the predetermined settling rate limit frommain fluid outlet 24 as finished slurry product. Line 30 conducts thefinished slurry product at a velocity in excess of saltation velocity toa liquefaction reactor 31 for further processing to produce liquidhydrocarbon products in a coal conversion operation. In the preferredembodiment illustrated, a portion of the liquid carrier in the slurryproduct is withdrawn from the slurry in line 30 and recycled by line 32back to the slurry drier 17 through a heater 33 to assist in themaintenance of the hot heat transfer pool.

Having now described the invention in detail, and having disclosed thepreferred manner in which the invention may be employed, what is desiredto be obtained in Letters Patent is described in the appended claims.

I claim:

1. A process of preparing a slurry of coal particles for plug freehandling in a slurry-handling system, which comprises, continuously:

charging said slurry as new feed to a settling-rate separation zonewhere coal particles settling at a velocity greater than a predeterminedsettling-rate limit are separated and constituted in an unfinished heavyslurry stream from other coal particles which at maximum settlingvelocity do not exceed the predetermined settling-rate limit, such othercoal particles being constituted in a finished slurry stream product forissuance from said separation zone, transferring said unfinished heavyslurry stream to a comminution zone where at least a portion of the coalparticles in the heavy slurry stream are reduced to a size, which forthe specific gravity of such particles, provides such particles with amaximum settling velocity no greater than said predetermined limit whenin said separation zone,

passing the comminuted heavy slurry stream to a mixing zone where thecoal particles in the comminuted heavy slurry stream are admixed withcoal particles in new feed slurry, and

charging the admixed slurry to said separation zone where the comminutedcoal particles which do not settle at a maximum velocity in excess ofsaid predetermined limit are constituted in said finished slurry streamproduct, and the comminuted coal particles and new feed coal particleswhich settle at rates in excess of said predetermined limit areconstituted in said unfinished heavy slurry stream,

recycling said unfinished heavy slurry stream to said comminution zonefor comminution, and discharging said finished slurry stream productinto said slurry handling system. 2. The process of claim 1 in whichsaid predetermined settling rate limit is within the range from about0.5 to about 300 ftJminute.

3. The process of claim 1 in which the slurry charged to said separationzone has a slurry specific gravity within the range from about 0.95 toabout 1.4 at 60 F.

4. The process of claim 1 in which said separation zone is ahydrocyclone.

5. The process of claim 1 in which said comminution zone is a ball mill.

6. The process of claim 1 in which the new slurry feed is prepared bycrushing coarse coal to a desired size in a sizing crusher which blowsthe crushed coal into a surge bin therebelow,

exhausting air transported into the surge bin with the crushed coal intoa gas cyclone above said surge bin to separate any particulate coalsolids entrained in the air,

recycling the air from the gas cyclone to the crusher whereby thequantity of fresh air in the crusher is reduced and oxidation of coalduring the crushing operation is reduced.

2. The process of claim 1 in which said predetermined settling ratelimit is within the range from about 0.5 to about 300 ft./minute.
 3. Theprocess of claim 1 in which the slurry charged to said separation zonehas a slurry specific gravity within the range from about 0.95 to about1.4 at 60* F.
 4. The process of claim 1 in which said separation zone isa hydrocyclone.
 5. The process of claim 1 in which said comminution zoneis a ball mill.
 6. The process of claim 1 in which the new slurry feedis prepared by crushing coarse coal to a desired size in a sizingcrusher which blows the crushed coal into a surge bin therebelow,exhausting air transported into the surge bin with the crushed coal intoa gas cyclone above said surge bin to separate any particulate coalsolids entrained in the air, recycling the air from the gas cyclone tothe crusher whereby the quantity of fresh air in the crusher is reducedand oxidation of coal during the crushing operation is reduced.